Combine the yeast, ¼ cup of the sugar, and lukewarm water in a large bowl, and allow to proof. Stir in the remaining ¼ cup of sugar, salt, milk, sour cream, lemon juice, and vanilla and mix well. Add the egg yolks and blend. With your fingertips, rapidly work the butter into 5 cups of the flour as you would for pie dough, to produce a dry, meal-like consistency. Add this to the yeast mixture and begin to knead in the bowl, adding more flour if necessary, to make a smooth, elastic dough. Turn out on a lightly floured board and knead 5 to 6 minutes. Shape into a ball and place in a lightly buttered bowl, turning to coat the surface with butter. Cover tightly and refrigerate to let rise for at least 4 hours or until doubled in bulk. Remove from the refrigerator, punch down, and turn out on a lightly floured board. (The dough can be kept in the refrigerator for as long as 3 days, in which case it should be punched down twice a day, until ready to roll out.)

Divide the dough in half, and roll out each piece into a rectangle about 10× 14 inches. Brush each rectangle with melted butter, and sprinkle with the brown or white sugar and cinnamon. Over this sprinkle the drained currants and then the finely chopped nuts. Gently press the filling into the dough with the rolling pin. Roll up from the wide end, jelly-roll fashion. Heavily butter two 9-inch tube pans. Carefully fit the rolls into the pans so that the ends of the dough join. Cover and let rise until doubled in bulk. Bake in a preheated 375° oven 45 to 55 minutes, until they are golden brown and give off a hollow sound when rapped with the knuckles. Let cool for 15 minutes in the pans, then invert on a rack. Meanwhile, melt the apricot jam over low heat. Add the brandy, Cointreau, or Grand Marnier, and blend. Strain, and coat the sides and top of the cakes with the glaze while the cakes are still warm. Cool them thoroughly before slicing.